The binary fraction of extreme horizontal branch stars

Abstract

We have used precise radial velocity measurements of subdwarf-B stars from the Palomar–Green catalogue to look for binary extreme horizontal branch (EHB) stars. We have determined the effective temperature, surface gravity and surface helium abundance for 20 of the targets from new or existing blue spectra and have compiled published values for these quantities for all but one other. We identify 36 EHB stars in our sample and find that at least 21 of these stars are binaries. All but one or two of these are new identifications. The minimum binary fraction for EHB stars implied by our survey is 60±8 per cent. Our survey is sensitive to binaries with orbital periods P≲10 d. For reasonable assumptions concerning the period distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbital periods is 87 per cent. Allowing for this estimated detection efficiency, the fraction of EHB stars that are short-period binaries (0.03 d≲P≲10 d) is 69±9 per cent. The value is not strongly dependent on the period distribution below P≈10 d or the mean companion mass for these short-period binaries. The orbital separation of the stars in these binaries is much less than the size of the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars, the orbital periods of which are ≳10 d, the fraction of EHB stars for which evolution was affected by the presence of a companion may be much higher, e.g. if one third of EHB stars are binaries with orbital periods 10 d≲P≲100 d, then our observations are consistent with all EHB stars being formed through some type of binary star evolution. We find that five of the other stars we observed are likely to be post-EHB stars, one of which is also a binary.